1. Field of the Disclosure
The embodiments disclosed herein relate to methods for producing ion exchanged glass, especially such glass with characteristics of moderate compressive stress, high depth of compressive layer, and/or desirable central tension.
2. Related Discussion
Glass laminates can be used as windows and glazing in architectural and vehicle or transportation applications, including automobiles, rolling stock, locomotive and airplanes. Glass laminates can also be used as glass panels in balustrades and stairs, and as decorative panels or coverings for walls, columns, elevator cabs, kitchen appliances and other applications. As used herein, a glazing or a laminated glass structure is a transparent, semi-transparent, translucent or opaque part of a window, panel, wall, enclosure, sign or other structure. Common types of glazing that are used in architectural and/or vehicle applications include clear and tinted laminated glass structures.
Conventional automotive glazing constructions may consist of two plies of 2 mm soda lime glass with a polyvinyl butyral (PVB) interlayer. These laminate constructions have certain advantages, including, low cost, and a sufficient impact resistance for automotive and other applications. However, because of their limited impact resistance and higher weight, these laminates usually exhibit poor performance characteristics, including a higher probability of breakage when struck by roadside debris, vandals and other objects of impact and lower fuel efficiencies for a respective vehicle.
In applications where strength is important (such as the above automotive application), the strength of conventional glass may be enhanced by several methods, including coatings, thermal tempering, and chemical strengthening (ion exchange). Thermal tempering is commonly used with thick, monolithic glass sheets, and has the advantage of creating a thick compressive layer through the glass surface, typically 20 to 25% of the overall glass thickness. Disadvantageously, however, the magnitude of the compressive stress is relatively low, typically less than 100 MPa. Furthermore, thermal tempering becomes increasingly ineffective for relatively thin glass, such as less than about 2 mm.
In contrast, ion exchange (IX) techniques can produce high levels of compressive stress in the treated glass, as high as about 1000 MPa at the surface, and is suitable for very thin glass. Disadvantageously, however, ion exchange is limited to relatively shallow compressive layers, typically on the order of tens of micrometers or so. The high compressive stress may result in very high blunt impact resistance, which might not pass particular safety standards for automotive applications, such as the ECE (UN Economic Commission for Europe) R43 Head Form Impact Test, where the glass is required to break at a certain impact load to prevent injury. Conventional research and development efforts have been focused on controlled or preferential breakage of vehicular laminates at the expense of impact resistance thereof.
Although the conventional single step ion exchange processes may employ a long ion exchange step to achieve a higher depth of compressive layer (DOL), such lengthy durations also result in a rise in the central tension (CT) past a chosen frangibility limit of the glass, resulting in undesirable fragmentation of the glass. By way of example, it has been newly discovered by experimentation that a 4 inch×4 inch×0.7 mm sheet of Corning® Gorilla Glass® will, upon fracture, exhibit undesirable fragmentation (energetic failure into a large number of small pieces) when a long single step ion exchange process (8 hours at 475° C.) has been performed in pure KNO3. Indeed, although a DOL of about 101 μm was achieved, a relatively high CT of 65 MPa results, which was higher than the desired frangibility limit (48 MPa) of the subject glass sheet.
Further, it has been newly discovered that installed automotive glazing (using ion exchanged glass) may develop external scratches as deep as about 75 μm due to exposure to environmental abrasive materials such as silica sand, flying debris, etc. This depth will exceed the typical depth of compressive layer (e.g., a few tens of micrometers), which could lead to the glass unexpectedly fracturing.
In view of the foregoing, new methods and apparatus are needed to address certain glass applications, where moderate compressive stress, high depth of compressive layer, and/or desirable central tension are important considerations.